1. Field of the Invention
The present invention relates to techniques of measuring the optical power of light and, more particularly, to a method and apparatus for measuring the optical power of very weak light and an optical communication system using the method.
2. Description of the Related Art
In the field of optical communications, in recent years, active studies have been devoted to quantum key distribution systems, which are regarded as realizing the high secrecy along a transmission line, and a variety of proposals have been made in regard to the systems.
As a basic one of the systems, a system which allows a sender and a receiver to share a quantum cryptographic key by using two types of bases is proposed in Bennett and brassard, “QUANTUM CRYPTOGRAPHY, PUBLIC KEY DISTRIBUTION AND COIN TOSSING” IEEE International Conference on Computers, Systems, and Signal Processing, Bangalore, India, pp. 175-179. According to this proposal, the sender phase-modulates a photon by using any one of four kinds of information according to the combinations of two bases (D, R) each representing quantum states, and binary random data (0, 1), and then transmits the phase-modulated photon to the receiver. The receiver receives the photon by using the bases (D, R) independently of the sender and stores the reception data. Thereafter, the sender and receiver use an ordinary channel to check whether or not the bases used in transmission and the bases used in reception are the same, and determine final shared secret data based only on the reception data corresponding to the matching bases.
A plug and play quantum key distribution system proposed by a team of the University of Geneva, Switzerland (see G. Ribordy, “Automated ‘plug & play’ quantum key distribution”, Electronics Letters, Vol. 34, No. 22, PP. 2116-2117) is thought of as a promising scheme for bringing polarization-sensitive quantum key distribution systems into practical use, because this system can compensate for the fluctuation in polarization occurring along an optical fiber transmission line. A schematic configuration of a plug and play system is shown in FIG. 1.
As shown in FIG. 1, in the plug and play system, a receiver, the one to receive a quantum cryptographic key, is provided with a laser LD, which generates optical pulses P. An optical pulse P is split into two parts at an optical coupler, and one of the two parts, an optical pulse P1, passes along a short path, whereas the other one, an optical pulse P2, passes along a long path. The pulses P1 and P2 are transmitted to a sender as double pulses.
The sender is provided with a Faraday mirror and a phase modulator A. The received optical pulses P1 and P2 are reflected by the Faraday mirror individually, whereby they are sent back to the receiver with their polarization states rotated by 90 degrees. In this event, the phase modulator A phase-modulates the optical pulse P2 at the timing when the optical pulse P2 is passing through the phase modulator A. The phase-modulated optical pulse P2*a is returned to the receiver.
Since the polarization state of each of the optical pulses P1 and P2*a received from the sender has been rotated by 90 degrees, a polarization beam splitter PBS in the receiver leads each of these received pulses into the other path that is different from the path the pulse used when it was transmitted to the sender. Specifically, the received optical pulse P1 is led into the long path and phase-modulated at the timing when it is passing through a phase modulator B, and the phase-modulated optical pulse P1*b arrives at the optical coupler. On the other hand, the optical pulse P2*a, which has been phase-modulated at the sender, passes along the short path, which is different from the path it used when transmitted to the sender, and then arrives at the same optical coupler. Accordingly, the optical pulse P2*a, phase-modulated on the sender side, and the optical pulse P1*b, phase-modulated on the receiver side, interfere with each other, and the result of this interference is detected by a photon detector APD0 or APD1. Note that avalanche photodiodes are used as the photon detectors, which are driven in a gated Geiger mode (GGM).
As described above, one optical pulse generated at the receiver is split into two, and the thus obtained double pulses P1 and P2 each have a round-trip between the receiver and sender while being phase-modulated individually. As a whole, the two pulses pass along the same optical path and then interfere with each other. Accordingly, the result of the interference observed by the photon detector APD0 or APD1, in which variations in delay due to an optical fiber transmission line are cancelled out, depends on the difference between the amount of a phase modulation at the sender and the amount of a phase modulation at the receiver.
A plug and play system having such a configuration requires synchronization as cited below.
(1) In the sender, it is necessary to apply a voltage corresponding to the amount of a phase modulation to the phase modulator A synchronously with the timing when the optical pulse P2 transmitted from the receiver is passing through the phase modulator A.
(2) In the receiver, it is necessary to apply a voltage corresponding to the amount of a phase modulation to the phase modulator B synchronously with the timing when the optical pulse P1 returned from the sender is passing through the phase modulator B.
(3) Further in the receiver, it is necessary to apply bias to the photon detectors APD0 and APD1 synchronously with the timing of the incidence of the returned pulse (supersensitive reception in the gated Geiger mode).
As described above, for a quantum key distribution system to stably generate a quantum cryptographic key by achieving high interference in practice, it is indispensable to perform such timing control that the phase modulator A on the sender side and the phase modulator B and photon detectors APD on the receiver side are each driven synchronously with the timing of the arrival of an optical pulse.
In the case of a system that transmits information by utilizing phase modulation such as the above-described quantum key distribution system, it cannot be determined whether or not the timing of driving the phase modulator A at the sender is right, unless the result of the interference observed by the photon detector APD0 or APD1 on the receiver side is referred to. Accordingly, in order to accurately perform the above-mentioned timing control, it is necessary to accurately know the extinction ratio of the interferometer (the ratio between the optical powers observed by the photon detectors APD0 and APD1) at the receiver.
However, in the case of the gated-Geiger-mode reception, bias is applied to a photon detector APD only for a predetermined period of time, at the timing of the arrival of an optical pulse. If a photon arrives while the gate voltage is being applied, the photon detector APD is broken down, and multiplication current continues to flow until the application of gate voltage is finished. Therefore, what can be detected by the photon detector is, except a noise, merely whether or not a photon arrives during the gate voltage application, and the extinction ratio, which is obtained based on the time average of photon detections, cannot be measured.
Therefore, it has been conventionally necessary that, each time transmission paths are changed, the extinction ratio be measured by using, for example, optical power measurement equipment in place of the photon detectors APD and then the timing of applying voltage to the phase modulator at the sender be determined. In other words, it has been necessary to provide the receiver with both the optical power measurement equipment for determining the sender's driving timing and the photon detectors APD for quantum cryptographic key generation.